Optical fibers are widely used in various communications systems. They offer an alternative to copper wires as a transmission medium, providing numerous advantages over copper technology. These advantages are as follows:
(a) very low signal loss PA1 (b) extremely wide bandwidth PA1 (c) no electromagnetic emission PA1 (d) not susceptible to interference PA1 (e) small physical size PA1 (f) low weight. PA1 1. Following fusion splicing, the fiber is very brittle--thus it must be protected. PA1 2. The fiber is susceptible to breakage between the time it is fused together and the time a protective splint (splice pack) is applied to it. PA1 3. The fusion splicing machine is a complicated, high precision, delicate instrument, which is very expensive. PA1 4. The quality of the fusion is dependent on factors such as electrode spacing, electrode shape, electrode cleanliness etc. These factors are influenced by maintenance and equipment care. PA1 5. The quality of the fusion splice is also influenced by the mechanical aspects of the splicing machine, such as the V-groove cleanliness, condition of the V-groove etc. PA1 6. Splice quality is also influenced by environmental conditions such as temperature, altitude and humidity, due to their effect on the fusion arc. PA1 1. The fiber ends are permanently joined, thus there is no light reflection at the joint. PA1 2. With the fiber ends permanently joined, long term splice quality is maintained, since no movement of the splice interface is possible. PA1 3. Activation of the splice with the electrical heating element may require only a low voltage, high current battery. Alternatively self-contained electrodes with a high voltage external power supply may be used. In either case the process is safer and simpler to operate. PA1 4. Fiber alignment is provided automatically. No precision V-grooves which may become damaged, misaligned or contaminated, are required. PA1 5. With the heat source self-contained in the splice unit and isolated from the outside environment, external influences such as altitude, humidity, wind, etc. will have no effect on the splice. PA1 6. The unit is relatively straightforward to manufacture and the installation tool (crimper) is low in cost. PA1 7. The design of the unit makes it unnecessary to apply further splice protection in order to achieve the necessary ruggedness. Thus a very small splice is achievable. PA1 8. The simplicity and small size of the unit allows its used in very tight spaces. PA1 9. The unit lends itself readily to a portable crimper tool which can be used to provide the mechanical/fused joint on fiber ends. PA1 10. With the fusion environment fully controlled, virtually no adjustment of the crimper will be required.
Because of the minute physical dimensions of the fibers (for example they may have an outer diameter of 5/1000"), two fibers which are to be spliced must be aligned very accurately so that their cores (the central, light conducting portions) are concentric.
Several techniques for joining fibers have been developed, two of which are widely used currently.
One of these, fusion splicing, relies of melting the fibers which are to be joined, until the two fibers flow into one another. Core joins to core and jacket (or cladding) to jacket.
This is accomplished by aligning the two fibers with high precision V grooves, bringing the cut ends of the fibers together until they touch, and surrounding the cut with an electric arc generated by a high voltage power supply. The arc generates very high temperatures, melting the fibers together.
Advantages of fusion splicing include the facts that the fibers essentially become integral with one another and hence the index of refraction, even where the fibers are joined, is homogeneous, eliminating back reflection of light. As well, due to the integral nature of a fusion spliced fibre, temperature changes or vibration do not affect the splice. Long term performance of fusion splices is now well established and they are known to be very reliable.
On the other hand, several disadvantages are inherent with fusion splicing. These include:
A second common technique for joining fibers is called "mechanical splicing". Mechanical splicing works on the principle of bringing the cut ends of the fibers together by some precision alignment tube, and holding them in that position by some mechanical means.
Several types of mechanical splices are on the market today, with names such as elastomeric splice, rotary splice, FIBRLOK* etc. FNT *=trade-mark
The elastomeric splice consists of a glass tube (capillary) into which a hollow elastic tube is pressed. The inside diameter of the tube is smaller than the diameter of the fibre, so that as the fiber is pushed into the hole, the force of the elastic material tends to center the fibers. Alignment of the fibers which are to be joined is thus achieved.
The elastic tube also has some optically transparent, viscous material injected into it, whose index of refraction is close to that of the optical fibers, thus reducing light reflection at the junction.
An overall mechanical splint is used around the splice, in order to keep the fibers from pulling apart.
The rotary splice can almost be described as an optical connector, in which each of the two fibers to be joined is glued into a ferrule. The ferrule ends are then polished and then the two are brought together in an alignment sleeve.
An overall mechanical housing holds the two ends together under spring tension.
The Fibrlok* splice is essentially similar to the others, except that the two fibers are surrounded by soft metal sleeves which are deformed by a tool. The deformed material exerts pressure on to the fibers, preventing them from moving. FNT * trade-mark
Advantages of mechanical splicing include the fact that the equipment required to install the splices is simple, the splice process is not affected by environmental conditions, no significant maintenance is required for the equipment, capital investment is low, there is a relatively low level of operator training required and equipment is small, allowing the user to perform splicing in cramped areas.
Disadvantages of mechanical splicing however are significant. Firstly, since the fibers are not physically joined into a single piece, vibration, long term creepage or material degradation may eventually degrade splice quality. Secondly, due the space between the two fiber ends (regardless how small that space is), reflection of light will occur, degrading performance.
Mechanical splicing systems are described in U.S. Pat. Nos. 4,820,007 of Ross et al, 4,892,381 of Glasheen and 5,011,825 of Takeda. An apparatus for fusing together optical fibers is described and illustrated in Thorncraft et al U.S. Pat. No. 5,011,252.
U.S. Pat. No. 3,810,802 of Buhite et al describes and illustrates and optical fiber splicing device and technique in which fibers are aligned collinearly in a hollow sleeve with a quantity of low melting point transparent thermoplastic glue being inserted in the sleeve at the junction of the two fibers. Heat is applied to melt the thermoplastic causing it to flow around the aligned ends, thereby producing an optically efficient bond with removal of the heat source.